The present invention relates to an image forming method in which an electrostatic latent image is formed on an image retainer of an electrostatic recording apparatus such as an electrophotographic copying apparatus, the formed electrostatic latent image is developed into a visualized image (toner image), and the visualized image is transferred onto a transfer sheet.
In an image forming apparatus in which an electrophotographic method or an electrostatic recording method is used, an electrostatic latent image is formed on an image retainer and the image is developed by toner which is a charged particle. The above-described principles are used in order to obtain a color image or a composite image (a plurality of originals or an image information and an original image are superimposed) in the following manner. That is, the image is obtained when a series of operations of charging, exposing and developing are repeatedly conducted not less than twice on an image retainer composed of a conductive base plate having a photoconductive layer thereon (for example, refer to U.S. Pat. No. 4,599,285).
The image forming method can develop a color image or perform composite images, and the superimposed toner images can be transferred onto a transfer sheet by a single transfer process, so that the apparatus, in which a color image or a composite image can be obtained, can be structured simply.
As a developing method by which the above-described image forming process can be accomplished, it is required to develop the image under the conditions disclosed, for example, in U.S. Pat. No. 4,557,992 or Japanese Patent Publication Open to Public Inspection No. 52565/1987 by using developing agent composed of non-magnetic toner and magnetic carrier. In a developing unit, developing agent is conveyed in the following manner: the developing agent is stirred to charge the toner, for example, negatively; the charged toner is adhered onto a magnetic carrier surface by electrostatic force, as described above; the developing agent, in which toner is combined with carrier electrostatically, is magnetically attracted onto the surface of a developing sleeve and is kept on the surface; and the developing agent is conveyed to a developing area while the developing sleeve is rotated at a predetermined line speed. The developing method is one of magnetic brush developing methods, and it is characterized in that: the magnetic brush is not contacted with the image retainer; and only toner is attracted to a latent image on the image retainer by a D.C. bias voltage or superimposing an A.C. bias voltage onto a D.C. bias voltage.
As an example of an image forming apparatus to which the image forming process and developing method, as described above, are applied, there exists an color image forming apparatus in which a latent image is formed at each color by a latent image forming method, and the image is developed by a developing unit containing color toner corresponding to each latent image.
In this kind of an image forming apparatus, the apparatus in which a ray of laser light or the like is irradiated on an image retainer (hereinafter, called a photoreceptor) having photoconductive material on a conductive base plate in order to form a latent image, is a typical one. Further, there is a method in which a tip of an LED is turned ON/OFF by a shutter such as a liquid crystal instead of a laser.
In such an image forming apparatus, a color toner image is formed in the toner image forming process shown in FIGS. 3(a) to 3(f).
FIGS. 3(a) to 3(f) are illustrations showing a toner image forming process in which toner images are superimposed on the photoreceptor after repeating charging, image exposing and developing.
As shown in FIG. 3(a), the photoreceptor is uniformly charged by the method of corona discharging by a charger, and the surface potential of the photoreceptor becomes uniform surface potential VH.
A laser exposing unit irradiates a laser beam so that the latent image can be formed on the surface of the photoreceptor. At this time, the surface potential of the photoreceptor on which a latent image is formed, is lowered from surface potential VH to VL1 as shown in FIG. 3(b). Surface potential VL1 of the photoreceptor is the surface potential of the electrostatic latent image formed at the first step, and is called exposure potential VL1. The surface potential of the electrostatic latent image formed on the surface of the photoreceptor is determined by an amount of laser light. Due to the aforementioned, development potential gap VG1 is generated between surface potential VDC of the developing sleeve due to a D.C. component of the bias voltage applied from a development bias circuit of the developing unit and exposure potential VL1. This development potential gap VG1 contributes to the following development in the following manner: electric field due to this developing gap VG1 goes from the surface of the latent image on the surface of the photoreceptor to the surface of the developing sleeve, and therefore toner, which is a negatively charged particle, is attracted by electric force going to a portion of the latent image on the surface of the photoreceptor. However, the electric force is not large enough to separate the toner including electrostatically combined charged particles, from the magnetic carrier. On the other hand, the developing agent which is kept on the surface of the developing sleeve by magnetic force, is subjected to further force when an A.C. component of bias voltage applied from the development bias circuit has the same polarity as a D.C. component, so that the toner flies towards an exposure portion with an exposure potential V.sub.L1 of the photoreceptor from the developing sleeve and adheres to the electrostatic latent image by electrostatic force. In this way, as shown in FIG. 3(c), the latent image on the photoreceptor is developed into the first toner image in the following manner that the toner including negatively charged particles is electrostatically adhered to the electrostatic latent image and developed. At this time, surface potential of the first toner layer adhered to the electrostatic latent image on the surface of the photoreceptor is equal to toner layer surface potential VT1.
The surface of the photoreceptor on which a toner layer is formed in the aforementioned first development process, is uniformly re-charged by the method of scorotron discharge with the charger, so that the surface potential of the photoreceptor becomes surface potential VH and the photoreceptor is ready for the next latent image formation. At this time, in the potential distribution of the surface of the photoreceptor as shown in FIG. 3(d), the surface is almost uniformly charged, and surface potential of the photoreceptor is VH.
The second image exposure is conducted by the laser exposing unit and the second latent image is formed on the photoreceptor surface. FIG. 3(e) shows the potential distribution of the photoreceptor surface at this time as follows. The surface potential of the photoreceptor at the first image exposure is lowered from VH to exposure potential VL1 as described above, the surface potential of the photoreceptor at the second image exposure is lowered from VH to re-exposure potential VL2, and the toner layer surface potential which is formed on the photoreceptor surface is lowered from the vicinity of VT2 to VT2. At this point, development potential gap VG1 which is the potential difference between surface potential VDC of the developing sleeve and exposure potential VL2 is generated, and development potential gap VG2 which is the potential difference between surface potential VDC of the developing sleeve and the re-exposure potential VL2 is generated. These development potential gaps VG1 and VG2 contribute to the next development. However, as described above, only the electric field due to these development potential gaps VG1 and VG2 does not generate enough electric force to separate the toner including electrostatically combined charged particles, from the magnetic carrier. At this point, also, when an A.C. component of the bias voltage applied from the development bias circuit, becomes the same polarity as the potential gaps due to VG1 and VG2, the force to convey the toner to the photoreceptor becomes large, so that the toner flies to the photoreceptor and adheres thereto. Due to the aforementioned, the latent image on the surface of the photoreceptor attracts the negatively charged toner electrostatically as shown in FIG. 3(f), so that the second toner layer is obtained by development.
After this process, a color toner image or a composite toner image is obtained on the photoreceptor by repeating the same process as the aforementioned by a required number of times. The toner image is transferred at one time onto a transfer sheet after the back of the transfer sheet has been charged to a reverse polarity to the toner by the transfer unit, and further the transfer sheet is heated or pressed for fixing, so that the composite image or color image can be obtained.
The aforementioned transfer of the toner image onto the transfer sheet is affected by the following conditions. When humidity becomes high, in the case of a transfer sheet which is difficult to obtain the uniform insulation property, its insulation property is lowered partially; and transition of electric charge is generated, so that the transferred charge is lost. As a result, transfer efficiency is partially lowered, and exceedingly uneven transfer, with the toner layer partially stripped off, is caused. In order to prevent the uneven transfer, a pre-transfer exposure method by which the entire surface is uniformly exposed just before transfer after all developments have been completed, is disclosed. When the pretransfer exposure is conducted, it is experimentally confirmed that the aforementioned uneven transfer can be prevented. However, toner particles themselves are charged in the manner of the same polarity as shown in FIG. 4, and therefore some toners are spattered around the edge portion of the toner image by an electrical repulsive force, the image becomes dirty, and resolution of the image is lowered, which are problems. This comes from the following reasons. The toner image is formed in an electric potential well at the development as shown in FIGS. 3(c) and 3(f), and therefore the electrical repulsive force caused by electric charges of toner still remains. When electric charge around the toner image is neutralized, the toner image portion has a protruded potential in contrast with the aforementioned, a portion of the toner is splashed around the toner image, and therefore the electrical repulsive force of the toner itself is released. Accordingly, the pre-transfer exposure method by which the entire surface of the toner image is uniformly exposed before transfer, does not become a perfect solution to the problem.
From the aforementioned reason, in order to prevent inferior transfer at the time of high humidity, it is required to provide a heater in the apparatus in order to prevent humidity rise of a transfer sheet and a transfer belt. Therefore there are problems in which the apparatus becomes complicated, or a specially treated precious transfer material should be used so that the transfer material is not affected by humidity.